This invention relates to liquid crystal display devices and more particularly to a field effect type liquid crystal display device having excellent time-division driving characteristics and being capable of monochromatic or black and white display.
FIG. 2 schematically illustrates a prior art liquid crystal display device having excellent time-division driving characteristics realized hitherto. As shown in FIG. 2, a nematic liquid crystal 3 having positive dielectric anisotropy is sealingly disposed in a gap between two electrode substrates 1 and 2, with liquid crystal molecules oriented as depicted roughly. The liquid crystal molecules are twisted over the gap between the substrates by an angle which exceeds 90.degree. , preferably, falls within a range of from 180.degree. to 270.degree. to have a twisted helical structure. Polarizer plates 4 and 5 are disposed exteriorly of the electrode substrates 1 and 2, with their polarization axes (absorption axes) oriented to make a suitable angle to a reference direction to be described later. Typically, this angle falls within a range of from 20.degree. to 70.degree..
The liquid crystal display device of the above construction is detailed in U.S. Pat. No. 4,443,065 or an article by D. J. Sheffer entitled "24.times.80 Character LCD Panel Using the Supertwisted Birefringence Effect", SID, 1985 Digest, pp. 120-123.
In order to obtain the twisted helical structure wherein the liquid crystal molecules are twisted over the gap between the two electrode substrates by an angle falling within the range of from 180.degree. to 270.degree., a so-called rubbing process may exemplarily be employed by which one surface, contiguous to the liquid crystal, of each electrode substrate is rubbed using a cloth in one direction. The liquid crystal molecules are oriented in the same directions as rubbing directions 8 and 9 in this rubbing process. The two electrode substrates 1 and 2 thus treated for orientation are disposed opposingly to provide the gap, with their rubbing directions 8 and 9 intersecting with each other to subtend an angle ranging from about 180.degree. to about 270.degree., and these electrode substrates 1 and 2 are bonded together using seal agent. When the nematic liquid crystal 3 having positive dielectric anisotropy is sealingly filled in the gap, the liquid crystal molecules are rotated and oriented over the gap between the electrode substrates by an angle which ranges from about 180.degree. to 270.degree. to have a twisted helical structure. The polarizer plate 4 is disposed above the substrate 1 and the polarizer plate 5 is disposed below the substrate 2. In order to optimize contrast, it is necessary that the polarization axis (or absorption axis) 6 or 7 of each polarizer plate make, clockwise or counterclockwise, an angle of 0.degree. to 70.degree. to the orientation direction of liquid crystal molecules contiguous to each electrode substrate.
In the example of FIG. 2, a backlighting source 10 is provided to carry out the transmission mode operation but a reflection plate may be provided in place of the backlighting source 10 to perform the reflection mode operation in accordance with substantially the same operational principle as that of the transmission mode.
In the liquid crystal display device of the above construction, voltage applied for display is related to luminance as graphically shown in FIG. 3. FIG. 3 demonstrates that in two modes, normally-open mode A and normally-closed mode B, the luminance steeply falls or rises in response to a voltage applied. Thanks to these characteristics, the high time-division driving can be ensured without degrading contrast.
The time-division driving will now be described in brief by taking dot matrix display, for instance. As shown in FIG. 4, stripe-shaped Y electrodes (signal electrodes) 12 are formed on the lower electrode substrate and stripe-shaped X electrodes (scanning electrodes) 11 are formed on the upper electrode substrate. Thus, a character or the like is displayed by turning on or off liquid crystal elements at cross points of the X electrodes and Y electrodes. In FIG. 4, sequential scanning of n scanning electrodes X.sub.1, X.sub.2,--X.sub.n is reiteratively repeated to perform the time-division driving. When a scanning electrode (X.sub.n in the Figure) is selected, some pixels and the remaining pixels on that scanning electrode are simultaneously applied with display selection signals and display non-selection signals according to a signal to be displayed through the signal electrodes 12 represented by Y.sub.1, Y.sub.2 --Y.sub.n. In this manner, cross points are selectively turned on or off using a combination of a scanning electrode 11 and voltage pulses applied to the signal electrodes 12. In this example, the number of scanning electrodes X corresponds to the number of time divisions.
Although the prior art liquid crystal display device called a supertwisted nematic type has the excellent time-division driving characteristics as described previously, it disadvantageously colors at least one of the background and display area as will be seen from FIG. 5 in which background color and display area color are indicated on CIE chromaticity coordinates, failing to perform monochromatic or black and white display.
In recent years, however, improvements in the quality of picture of liquid crystal display devices and an increase in the amount of information to be displayed have been urgently demanded, and specifications for monochromatic display and advanced up-to-date color display have not met demanded ones.
The conventional technique does not take it into account to make the background color white and the display area color black and is only permitted to provide limited display quality. Further, in the conventional technique, at least one of the background and display area is colored and therefore, color display can not be realized using a color filter in combination.